Water-absorptive resins which can absorb several hundred times their own weight of water can find wide applications such as absorbents used in paper diapers or sanitary products; water-retaining materials, soil-conditioners and water-stop materials for agricultural or horticultural use. As the water-absorptive resin, for example, there have been known starch-acrylonitrile graft copolymer hydrolysate; starch-acrylic acid graft copolymers; carboxycellulose crosslinked polymers; vinyl acetate-methyl acrylate copolymer hydrolyzate; and polyacrylic acid salt crosslinked polymers.
These water-absorptive resins have quite excellent absorption capacity for pure water and deionized water: however, they have a disadvantage that the absorption capacity is decreased for aqueous electrolyte solutions such as see water, brine, blood, sweat and urine. In particular, the absorption capacity is remarkably decreased for aqueous electrolyte solutions of high concentration, such as see water, or aqueous solutions of polyvalent metal salts, which causes some limitation of applications.
To increase the absorption capacity for aqueous electrolyte solutions, there have been proposed various salt-resistant absorbents, for example, crosslinked polymers containing sulfoalkyl(meth)acrylate (see JP-A 61-36309/1986) and crosslinked copolymers of a (meth)acrylate derivative having polyether side chains and a monomer having a sulfonic group (JP-A 62-266140/1987). However, these water-absorptive resins still have decreased absorption capacity for aqueous electrolyte solutions of high concentration, such as see water, or aqueous solutions of polyvalent metal salts.